1. Field of the Invention
The disclosed embodiments of the present invention relate to a calibration method and a related calibration circuit, and more particularly, to a calibration method and a related calibration apparatus for calibrating mismatches of an in-phase (I) signal path and a quadrature (Q) signal path of a transmitter/receiver.
2. Description of the Prior Art
In general, the modulation technique can comprise more information data when it is more complicated. That is, the modulation technique can increase transmission rate by complicated modulation process, such as the 64-Quadrature Amplitude Modulation (64-QAM), or even the 256-QAM. Thus, the requirement of high level orthogonal amplitude modulation is more and more popular. If the high level orthogonal amplitude modulation is expected to have good transmitting/receiving effect, the Error Vector Magnitude (EVM) of the communication system has to be increased accordingly. One of the important factor affecting the EVM is the level of the In-phase Quadrature-phase imbalance (IQ imbalance). The main cause of the IQ imbalance is the mismatch of IQ signal path of the Radio Frequency (RF) a first signal path and a second signal path of a receiver, and even a small bias will affect the whole communication system, to form an incomplete orthogonal modulation/demodulation procedure, and result in increase of Bit Error Rate (BER) of the receiving terminal. The bias can be separated to the amplitude bias and the phase bias. As long as the biases exist, the image interference of symmetrical frequency will be generated in the spectrum. Please refer to FIG. 1. FIG. 1 is a diagram illustrating a received signal received by a receiving terminal and an image signal generated by the received signal. The difference between the received signal amplitude and the image signal amplitude is called Image Rejection Ratio (IRR) in general. For example, when IQ imbalance is large, IRR will be small, and IQ imbalance is small, IRR will be large.
In order to improve the effect caused by the bias, a calibration will be performed for the circuit when transmitting/receiving signals formally, and the calibration is called IQ calibration. There are two main causes of mismatches of the IQ signal path. One of the main causes is when a local oscillator (LO) generates carrier waves and transmits the carrier waves respectively to the mixers in the I signal path and the Q signal path, it is hard to have a phase difference of 90 degrees, or the amplitudes of the carrier waves respectively transmitted to the mixers in the I signal path and the Q signal path are different (i.e., the above phase bias and amplitude bias). Another one of the main causes is that it is hard to avoid imperfect conditions in the circuit process, and thus two elements in the I signal path and the Q signal path are not totally matched to each other. For example, when there are mismatches between the low-pass filter (LPF), the analog-to-digital converter (ADC), the digital-to-analog converter (DAC, or the gain amplifier in the I signal path and the Q signal path, the signals passing by the I signal path and the Q signal path will have difference. When the signals pass by the IQ signal path with mismatches, the image interference signals will be generated (as shown in FIG. 1) and reduce the signal quality.
For the wideband communication system, the image signal interference for the signals in different frequencies within a frequency band is not always the same. In general, the prior art uses least mean square (LMS) method to find out the best compensation filter to calibrate the image interference signals in the frequency band range. However, the requirement for connecting speed is higher and higher in the wireless communication application nowadays. For example, if there is a phone call coming when a user uses a Bluetooth earphone, the Bluetooth earphone has to connect to the smartphone right away for user to take the phone call. The higher connecting speed can bring a better user experience. Thus, it is an important issue about how to perform the IQ calibration fast and accurately in the area of the communication system.